This invention relates to ion exchange, and more particularly, to ion exchange systems that are regenerated electrically.
The concept of electrically regenerating ion exchange resins has been actively investigated for more than twenty years, but previous systems using this approach have not appeared to be economically competitive with standard chemical processes for regenerating the resins. Early investigators apparently all attempted to electrically regenerate the strong-acid and strong-base resins that were commercially available. They achieved considerable deminerialization of aqueous solutions of electrolytes, but the quantities of resin and expenditures of electrical energy required made them uneconomical. For example, in batch regeneration experiments with mixed beds, described in "Concentration of Radioactive Aqueous Wastes," Ind. Eng. Chem., 47, 61-67 (1955), Walters, Weiser and Marek obtained 6.2% regeneration of the ion exchange capacity of their resins with 52% coulomb efficiency, i.e., 0.52 equivalents of salt were removed from the resin per Faraday of current passing through the resin bed, but by the time the resin was 14% regenerated the coulomb efficiency for the entire time period had dropped to 37% and the actual coulomb efficiency at the end of the regeneration was only 5%.
During steady-state experiments described in SOME EXPERIMENTAL STUDIES OF ELECTRODEIONISATION THROUGH RESIN PACKED BEDS, United Kingdom Atomic Energy Authority Report AERE-R 4517. ( 1964), Gittens and Watts removed 13.5% of the NaNO.sub.3 from a 10.sup.-.sup.3 M solution with a coulomb efficiency of 59%, but the coulomb efficiency dropped to 10% when they removed 28% of the salt from a 10.sup.- 4 M solution.
In a more recent study, ELECTROLYTIC REGENERATION OF ION EXCHANGE RESINS, Final Report to the Office of Saline Water on Contract 14-01-0001-1255 (1968), Prober and Myers obtained regeneration rates greater than 1.0 equivalent per Faraday per resin bed in some experiments, but their efficiencies were much lower than that for most cases. They concluded that electroregeneration was technically feasible, but did not appear to be economically feasible.
A report by the Electric Storage Battery Company to the Office of Saline Water, INVESTIGATION AND DEVELOPMENT OF AN ELECTROLYTIC SYSTEM FOR THE CONVERSION OF SALINE AND BRACKISH WATERS, O.S.W. Research and Development Progress Report No. 51, (1961), suggests that electrically regenerated ion exchange systems might be made more efficient or economical by using rods of ion exchange material or membranes extending between an anode chamber and a cathode chamber. Membrane type mixed bed systems of this sort were operated successfully, but the report concluded that the energy consumption did not compare favorably with established processes and that no means for improving the economics were evident.
In a study conducted for the Artificial Kidney-Chronic Uremia Program of the National Institute of Arthritis, Metabolism and Digestive Disease, discussed in ELECTRO-REGENERATION OF ION EXCHANGE RESINS, National Institutes of Health Report AK-2-70-2108-F (1972), Davis and Lacey demonstrated that electroregeneration can be quite efficient if resins are selected that have higher electrical conductivity in the exhausted form than in the regenerated form. With a mixture of weak-acid cation exchange resin and weak-base anion exchange resin, they achieved 99% demineralization of 0.002 N NaCl solutions in a continuous processes with over 50% coulomb efficiency and about 50 watt hr. of power consumed per equivalent of salt removed.